parted/LVM for ET [Re: [Libvir] Storage manager initial requirements and thoughts

[Richard W.M. Jones]

Richard W.M. Jones wrote:
> Richard W.M. Jones wrote:
>> lvcreate -L 3G -n newroot raidvg
>> lvcreate -L 1G -n newswap raidvg
> 
> I should add, in a libvirt context it's probably going to be useful to 
> also:
> 
> * list available volume groups (vgscan)
> * list space available in each VG (vgdisplay name-of-vg)
> * show how VGs relate to PGs (pvscan)
> 
> The LVM tools use quite a sophisticated internal API which isn't 
> exported - but see for example tools/tools.h and include/*.h in the LVM2 
> source code.  <libdevmapper.h> is also available, but much more lowlevel.

I talked to Alasdair Kergon about all of this on Tuesday, and there are 
a few clarifications.

Firstly, vgscan & vgdisplay are apparently deprecated (who knew?) and 
there is a new command (vgs) which replaces both.  The good news is that 
vgs is designed so that the output can be parsed:

# /usr/sbin/vgs
   VG         #PV #LV #SN Attr   VSize   VFree
   Home         1   1   0 wz--n- 298.09G  8.09G
   VolGroup00   1   2   0 wz--n- 152.56G 32.00M

versus the old way:

# /usr/sbin/vgdisplay Home
   --- Volume group ---
   VG Name               Home
   System ID
   Format                lvm2
   Metadata Areas        1
   Metadata Sequence No  2
  [etc]

Similarly, pvs instead of pvscan/pvdisplay and lvs instead of 
lvscan/lvdisplay.

Secondly there is an API of sorts for lvm2.  I think Alasdair called it 
"libcmd", but maybe I got that wrong because Google doesn't seem to turn 
up anything.  In any case, all it is is a wrapper around the command 
line tools, so it seems doubtful that this is going to be any better 
than just invoking the command line tools ourselves.

Thirdly, Alasdair is quite keen to see a proper programming API for 
LVM2.  However I don't think anyone has the time to do it, and he has 
the usual concerns for backwards binary compatibility and making sure 
the API mirrors the command line tools.  LVM2 is still evolving at a 
fair rate.  There are few other candidates to use the API at the moment, 
but apparently Anaconda does some really nasty stuff directly on LVM2 
structures so it should be changed to use an API instead.

My current opinion is that we can use libparted as it is to both manage 
virt-on-partition scenarios and to find orphan PVs as candidates to add 
to a VG, and the command line tools like vgs to manage and allocate LVM2 
partitions.

Rich.

-- 
Emerging Technologies, Red Hat  http://et.redhat.com/~rjones/
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